The present invention generally relates to feature extracting methods, and more particularly to a feature extracting method for extracting features from a line drawing in character recognition apparatuses, drawing recognition apparatuses and the like.
In order to efficiently process image data in apparatuses such as character recognition apparatuses and drawing recognition apparatuses, it is generally necessary to describe in an abstract form an original image data which is represented as a two-dimensional arrangement for the purpose of describing characteristics of the original image. A graph representation is a most popular method of describing a line drawing. The conventional methods of making the graph representation can be roughly divided into a thinning method and a vectorizing method. These methods may be found in Nakajima et al., "A Survey of Map and Drawing Data Input Technology", Shingakugiho, IE88-5, 1988.
According to the thinning method, a thinning process is carried out with respect to the entire image data by eroding black region from its boundary and strokes are detected by tracing black picture elements of the thinned image. The graph representation is obtained based on the detected strokes. This method has an advantage in that the graph representation can always be obtained regardless of the line width and resolution of the line drawing. But on the other hand, this method suffers from problems in that a large information quantity must be processed and much of the information is lost during the thinning process. As a result, unwanted distortions and creases are generated.
According to the vectorizing method, borders of lines are traced and a vector description is made of the borders. The graph representation is obtained by forming a vector in a portion between a confronting vector pair. This method has an advantage in that the information quantity which must be processed is relatively small. But on the other hand, this method suffers from problems in that it is difficult to automatically set parameters which determine the vector pair depending on the line width and the resolution, and that a desired skeleton cannot be obtained when short strokes are tightly placed such as the case of a character, a graphic symbol and the like.
Furthermore, according to the thinning method and the vectorizing method, there is a common problem in that it is difficult to absorb a deviation in the pattern such as smear and faint stroke.
The problems described above are mainly due to the fact that each of the conventional methods only use a bottom-up processing which uses the picture element or its limited adjacency as the observation unit.